The debate over whether video games cause aggressive behaviour in players has been going on for years. Every time there's a mass killing in the US, someone finds out that the perpetrator played some kind of video game, then the media goes crazy, saying that the video game made them violent.
A recent study has determined that yes, video games do cause aggression in players, but only when the game itself totally sucks. We've all played games that are frustrating: impossible puzzles, impervious bosses, idiotic gameplay, broken game components, and so on. Encountering frustration like that in a game does lead to aggression in players. Well, duh.